Among known oil pumps are a gear pump for feeding oil under pressure by two gears which are in intermeshing engagement, a trochoidal pump having an inner rotor and an outer rotor, etc.
The trochoidal pump comprises an inner rotor having internal teeth and an outer rotor having external teeth, the rotors being defined by a trochoid and incorporated within a housing section of a pump housing eccentrically to each other, the teeth of the inner rotor being one fewer in number than those of the outer rotor, and the outer rotor being adapted to rotate in the same direction of the inner rotor in association with the latter to provide variable volume spaces defined between the internal teeth and the external teeth so that fluid flows in and out through inlet and outlet ports. This trochoidal pump is, as compared with other kinds of oil pumps of same capacity, small in size, simple in construction, and less noisy while the teeth intermesh, and so has a wide application as a lubricating oil pump for vehicles or as an oil pump for automatic speed change gears.
The above construction of the trochoidal pump is, for example, disclosed by Japanese Patent Publication No. 47-33843. The disclosed device is a differential consisting of an oil pump constituted by a trochoidal pump and a motor, which device is compact in configuration, with the interval between the output shafts optionally expandable, and has a hydraulic circuit which is made narrow in its intermediate portion to provide an easy differential restriction, and additionally, this hydraulic circuit can be removed outside the differential gear to be attached to a control valve, thereby facilitating interruption of driving in the exchange of a differential gear, differential locking device.
As shown in FIGS. 7 and 8, with a trochoidal pump 12, the distance between the center i of the inner rotor 18 and the center o of the outer rotor 20 is E (amount of space between centers), and the respective coordinates are dependent upon the center i of the inner rotor 18 and the center P of a housing bore (pocket) of a pump-housing section which accommodates the outer rotor 20. In order to allow the rotation of the inner and outer rotors, there is required a determined distance or space T between the tip end 18b, of the tooth 18a of the inner rotor 18 and the tip end 20b of the tooth 20a of the outer rotor 20, and a determined distance B between the outer periphery 20c of the outer rotor 20 and the inner periphery 24c of the housing section 24 when the tip ends 18b and 20b are positioned directly opposed to each other.
The clearances formed when the inner rotor 18 is pressed against the outer rotor 20 in a vertical direction with the tip end 18b of the internal tooth 18a positioned opposite to the tip end 20b of the external tooth 20a are maximum (Tmax) and minimum (Tmin, which in this case is equal to zero) as the distance T. Additionally, if the difference between the inner diameter DP of the housing section 24 and the outer diameter DO of the outer rotor 20 is C, the distance E between centers is defined as a distance between the centers of the inner and outer rotors 18, 20 upon settings of the distance T=(Tmax+Tmin).times.0.5 and of the distance B/2=0.5.times.C (see FIG. 8).
Since the center i of the inner rotor 18 is stationary, the outer rotor 20 is displaced as its distance E between centers varies within the range restricted to the said distances, so as to become stable or vibrate at a certain position when the drive torque, discharge pressure, and entrapment pressure (or tolerances of the respective portions) are in a state of equilibrium.
In this state, however, interference occurs between the internal tooth 18a of the inner rotor 18 and the external tooth 20a of the outer rotor 20 (trochoid interference), causing irregular intermeshing between the internal tooth 18a of the inner rotor 18 and the external tooth 20a of the outer rotor 20, thus resulting in occurrence of pump noise (namely roaring). That is, as Tmin=0, if 0.5.times.B&gt;0.5.times.Tmax, by the movement of the center O of the outer rotor 20 (variation of the distance E between centers) the distance T (Tmax) is reduced to zero in FIGS. 7 and 8, thereby causing an occurrence of trochoid interference between the internal tooth 18a and the external tooth 20a with the consequential irregular intermeshing engagement causing inconvenience of production or pump noise.
The object of the present invention is to obviate the above inconvenience, and to provide a trochoidal pump wherein the center of the outer rotor is set to ensure that the distance between the opposed tip ends of the internal and external teeth is made larger than zero, so that occurrence of pump noise by the fluctuation of the outer rotor may be effectively reduced.
It is also an object of the present invention to prevent an occurrence of irregular intermeshing engagement between the tip end of the internal tooth of the inner rotor and the tip end of the external tooth of the outer rotor even when the outer rotor is vibrated during its rotation in accordance with the rotation of the inner rotor, thereby to effectively diminish the generation of pump noise as irregular intermeshing noise.
In order to attain this object, the present invention provides a trochoidal pump comprising an inner rotor and an outer rotor incorporated eccentrically to each other within a housing section of a pump housing, the inner and outer rotors having internal and external teeth defined by a trochoid and held in intermeshing engagement, characterized in that the center of the outer rotor is set to ensure that the distance defined between the tip ends of said internal and external teeth will be greater than zero when the tip end of the internal tooth of the inner rotor and the tip end of the external tooth of the outer rotor are positioned opposite to each other by the rotation of the outer rotor in association with the inner rotor.
In accordance with the construction of the present invention, since the center of the outer rotor is set to ensure that the distance between the tip ends will be greater than zero, independent from the fluctuation of the outer rotor, any interference can be avoided between the internal tooth of the inner rotor and the external tooth of the outer rotor, and regular intermeshing of the internal tooth and the external tooth may be achieved, thus resulting in a decrease in the pump noise.